Warn when toggling erc-nicks-mode in target buffers

* lisp/erc/erc-nicks.el (erc-nicks-mode, erc-nicks-enable): This local
module isn't autoloaded like most global modules, in part to dissuade
users from attempting to enable it by running M-x erc-nicks-mode RET.
However, rather than signal an error upon detecting such an attempt,
ERC should explain that the only supported means of activation is via
`erc-modules'.  This change does that and then enables it anyway,
despite the degraded experience.

lisp/erc/erc-nicks.el

@@ -480,6 +480,12 @@ Abandon search after examining LIMIT faces."
   "Uniquely colorize nicknames in target buffers."
   ((if erc--target
        (progn
+         (erc-with-server-buffer
+           (unless erc-nicks-mode
+             (erc--warn-once-before-connect 'erc-nicks-mode
+               "Module `nicks' must be enabled or disabled session-wide."
+               " Toggling it in individual target buffers is unsupported.")
+             (erc-nicks-mode +1))) ; but do it anyway
          (setq erc-nicks--downcased-skip-nicks
                (mapcar #'erc-downcase erc-nicks-skip-nicks))
          (add-function :filter-return (local 'erc-button--modify-nick-function)